When information is written onto a magnetic disc, the information is recorded in digital form. Digital squarewaves have vertical edges that a magnetic head can sense. Positive and negative transitions in the square wave, that is the edges, are recognized and recorded on the magnetic medium of the disc. When the magnetic head is used to read the data stored on the disc, the output is not in digital form because of the characteristics of the head and the magnetic medium.
It is frequently desirable to recreate the original digital signal initially written onto the magnetic disc. Therefore, the information read from the disc must be operated upon to approximate the original digital signal written onto the disc. The signal read from the disc may include analog signals with flat regions between peaks. If the locations where the analog signal crosses the zero reference line can be determined, the outline of the original digital signal can be approximated.
In reconstructing the digital signal, a zero-crossing detection device may be used to indicate when an applied signal crossed the base line. By this method, the signal transition edges could be detected and the original signal approximated. However, a flat region of the applied signal adjacent to the base line between peaks created an indeterminant output from the zero-crossing detector. This flattening of the applied signal along the base line is called "shouldering".
Prior solutions to this shouldering phenomena included terminating the detection of zero crossings by binary gating during the periods when the flat regions occured between the peaks of the applied signals. A binary signal was used to operate the binary gate and if the binary signal was in error, the operation of the gate was also in error causing single bit errors in the detector output. In addition, the binary gating generator typically had a long time delay associated with its electronics. Other electronic circuits within the equipment then required a matching time delay so that the desired output could be derived in timed sequence with the delayed binary signal.